/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* reverseList(struct ListNode* head) {
    if(head==NULL) return NULL;
    struct ListNode* n1=NULL;
    struct ListNode* n2=head;
    struct ListNode* n3=n2->next;
    while(n2)
    {
        n2->next=n1;
        n1=n2;
        n2=n3;
        if(n3) n3=n3->next;
    }
    return n1;
}
